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Is there one (or, one with slight modifications) diet that all humans should be eating? I’m skeptical of such claims. Though both vegans (one who does not eat or use animal products) and carnivores (one who eats only animal products), in my opinion, have some warped views on diet and human evolution. Both are extreme views; both have wrong ideas about diet throughout our evolution; both get some things right. Though, both are extreme views with little to no support. While it is hard to pinpoint what the “human diet” is, clearly, there were certain things that we ate through our evolutionary niches in our ancestral Africa that we “should” be eating today (in good quantities).

Although it is difficult to reconstruct the diet of early hominids due to lack of specimens (Luca, Perry, and Rienzo, 2014), by studying the eating behavior of our closest evolutionary relatives—the chimpanzees—we can get an idea of what our LCA ate and its eating behavior (Ulijaszek, Mann, and Elton, 2013). Humans have been throughout most every niche we could possibly been in and, therefore, have come across the most common foods in each ecology. If animal A is in ecosystem E with foods X, Y, and Z, then animal A eats foods X, Y, and Z, since animals consume what is in their ecosystem. Knowing this much, the niches our ancestors lived in in the past had to have a mix of both game and plants, therefore that was our diet (in differing amounts, obviously). But it is more complicated than that.

So, knowing this, according to Ulijaszek, Mann, and Elton, (2013: 35) “Mammalian comparisons may be more useful than ‘Stone Age’ perspectives, as many of the attributes of hominin diets and the behaviour associated with obtaining them were probably established well before the Pleistocene, the time stone agers were around (Foley 1995; Ulijaszek 2002; Elton 2008a).” Humans eat monocots (various flowering plants with one seed), which is not common our order. The advent of farming was us “expanding our dietary niche”, which began “the widespread adoption of agriculture [which] is an obvious point of transition to a ‘monocot world’” (Ulijaszek, Mann, and Elton, 2013). Although these foodstuffs dominate our diet, there is seasonality in what types of those foods we consume.

So since humans tend to not pick at things to eat, but have discrete meals (it is worth noting that one should have “three square meals a day” is a myth; see Mattson et al, 2014), we need to eat a lot in the times we do eat. Therefore, since we are large-bodied primates and our energy needs are much higher (due to our large brains that consume 20 percent of our daily caloric consumption), we need higher quality energy. The overall quality and energy density of our diets are due to meat-eating—which folivorous/frugivorous primates do not consume. We have a shorter gut tract which is “often attributed to the greater reliance of faunivory in humans“, though “humans are not confined to ‘browse’ vegetation … and make extensive use of grasses and their animal consumers” (Ulijaszek, Mann, and Elton, 2013: 58). Due to this, we show amazing dietary flexibility and adaptability due to our ability to eat a wide range of foodstuffs in most any environment we find ourselves in.

So “It is difficult to pinpoint what the human diet actually is … Nonetheless, humans are frequently described as omnivores” (Ulijaszek, Mann, and Elton, 2013: 59). Omnivores normally feed at two or more trophic levels, though others define it as just consuming plants and animals (Chubaty et al, 2014). Trophic level one is taken up by plants; level two is taken up by herbivores—primary consumers; level three is taken up by predators—who feed on the herbivores; level four or five is taken up by apex predators or carnivores; while the last level is also taken up by detrivores—those who feed on waste. Though, of course, “omnivory” is a continuum and not a category in and of itself. Humans eat primary producers (plants) and primary consumers (herbivores) and some secondary consumers (like fish), “although human omnivory may only be possible because of technological processing” (Ulijaszek, Mann, and Elton, 2013: 59). Other animals described as “omnivorous” eat only foods from one trophic level and only consume food from another level when needed.

Humans—as a species—rely on meat consumption. Fonseca-Azevedo and Herculano-Houzel (2012) showed that the energetic cost of a brain is directly related to the number of neurons in the brain. So, there were metabolic limitations in regard to brain and body size. The number of hours available to feed along with the low caloric yield of plant foods explains why great apes have such large bodies and small brains—which was probably overcome by erectus, who probably started cooking food around 1.5 mya. If we consumed only a diet of raw foods, then it would have taken us around 9 h/day to consume the calories we would need to power our brains—which is just not feasible. So it is unlikely that erectus—who was the first to have the human body plan and therefore the ability to run, which implies he would have needed higher quality energy—would have survived on a diet of raw plant foods since it would take so many hours to consume enough food to power their growing brains.

We can see that we are adapted to eating meat by looking at our intestines. Our small intestines are relatively long, whereas our long intestines are relatively short, which indicates that we became adapted to eating meat. Our “ability to eat significant quantities of meat and fish is a significant departure from the dietary norm of the haplorhine primates, especially for animals in the larger size classes.” Though “Humans share many features of their gut morphology with other primates, particularly the great apes, and have a gut structure that reflects their evolutionary heritage as plant, specifically ripe fruit, eaters” (Ulijaszek, Mann, and Elton, 2013: 63). Chimpanzees are not physiologically adapted to meat eating, which can be seen in the development of hypercholesterolemia along with vascular disease, even when controlled diets in captivity (Ford and Stanford, 2004).

When consuming a lot of protein, though, “rabbit starvation” needs to be kept in mind. Rabbit starvation is a type of malnutrition that arises from eating little to no fat and high amounts of protein. Since protein intake is physiologically demanding (it takes the most energy to process out of the three macros), Ben-Dor et al (2011) suggest a caloric ceiling of about 35 percent of kcal coming from protein. So erectus’ protein ceiling was 3.9 g/bw per day whereas for Homo sapiens it was 4.0 g/bw per day. Ben-Dor et al (2011) show that erectus’ DEE (daily energy expenditure) was about 2704 kcal, with “a maximum long-term plant protein ceiling of 1014 calories“, implying that erectus was, indeed, an omnivore. So, of course, the consumption of protein and raw plants are physiologically limited. Since erectus’ ceiling on protein intake was 947 kcal and his ceiling on raw plant intake was 1014 kcal, then, according to the model proposed by Ben-Dor et al (2011), erectus would have needed to consume about 744 kcal from fat, which is about 27 percent of his overall caloric intake and 44 percent of animal product intake.

Neanderthals would have consumed between 74-85 percent of their daily caloric energy during glacial winters from fat, with the rest coming from protein (Ben-Dor, Gopher, and Barkai, 2016), while consuming between 3,360 to 4,480 kcal per day (Steegman, Cerny, and Holliday, 2002). (See more on Neanderthal diet here.) Neanderthals consumed a large amount of protein, about 292 grams per day (Ben-Dor, Gopher, and Barkai, 2016: 370). Since our close evolutionary cousins (Neanderthals and erectus) ate large amounts of protein and fat, they were well-acclimated, physiologically speaking, to their high-protein diets. Though, their diets were not too high in protein to where rabbit starvation would occur—fat was consumed in sufficient amounts in the animals that Neanderthals hunted and killed, so rabbit starvation was not a problem for them. But since rabbit starvation is a huge problem for our species, “It is therefore unlikely that humans could be true carnivores in the way felids are” (Ulijaszek, Mann, and Elton, 2013: 66).

We consume a diet that is both omnivorous and eclectic, which is determined by our phylogeny through the form of our guts; we have nutritional diversity in our evolutionary history. We needed to colonize new lands and, since animals can only consume what is in their ecosystem, the foods that are edible in said ecosystem will be what is consumed by that animal. Being eclectic feeders made the migration out of Africa possible.

But humans are not true carnivores, contrary to some claims. “Meat-eating has allowed humans to colonize high latitudes and very open landscapes. However, bearing in mind the phylogenetic constraints that prevent humans from being true carnivores, such expansion was probably not accomplished through meat-eating alone. Instead, humans have used their ability to technologically harvest, produce, and consume a very wide range of foods to help exploit all major biomes” (Ulijaszek, Mann, and Elton, 2013: 67).

Humans, though, lack the gut specialization and dentition to process grasses efficiently. This means that our ancestors ate animals that ate these things, and therefore the C4 they consumed elevated the levels in the fossils we discovered. Information like this implies that our ancestors ate across a wide variety of trophic levels and had substantial dietary diversity throughout evolutionary history.

“Hominins lack the specialized dentition found in carnivorans (the group of animals that includes the cat and dog families) and other habitual meat and bone eaters, so must have pre-processed at least some of the meat in their diet” (Ulijaszek, Mann, and Elton, 2013: 81). This is where stone tools come into play (Zink and Lieberman, 2016). “Processing” food can be anything from taking out nutrients to changing how the food looks. We can look at “food processing” as a form of pre-digestion before consumption. The use of stone tools, and cooking, was imperative for us to begin the processing of meat and other foods. This gave us the ability to “pre-digest” our food before consumption, which increases the available energy in any food that is cooked/processed. For example, cooking denatures protein strands and breaks down the cell walls which gelatinizes the collagen in the meat which allows for easier chewing and digestion. Carmody et al (2016)showed that adaptation to a cooked diet began around 275 kya.

Raw-foodists are dedicated to eating 100 percent of their diets raw, or as close to 100 percent as they can manage. There are only three studies of their body weight, and all find that people who eat raw tend to be thin. The most extensive is the Giessen Raw Food study, conducted by nutritionist Corinna Koebnick and her colleagues in Germany, who used questionnaires to study 513 raw-foodists who ate from 70 to 100 percent of their diet raw. They chose to eat raw to be healthy, to prevent illness, to have a long life, or to live naturally. Raw food included not only uncooked vegetables and occasional meat, but also cold-pressed oil and honey, and some items were lightly heated such as dried fruits, dried meat, and dried fish. Body mass index (BMI), which measures weight in relation to the square of the height, was used as a measure of fatness. As the proportion of food eaten raw rose, BMI fell. The average weight loss when shifting from a cooked to a raw food diet was 26.5 pounds (12 kilograms) for women and 21.8 pounds (9.9 kilograms) for men. Among those eating a purely raw diet (31 percent), the body weights of almost a third indicated chronic energy deficiency. The scientists’ conclusion was unambiguous: “a strict raw food diet cannot guarantee an adequate energy supply.”

Also, vegetarians and meat-eaters who cooked their food have similar body weights. This implies that cooking food—no matter the type—gives more caloric energy to use for the body and that raw-foodists are fighting a losing battle with biology, consuming raw foods at such a high quantity that our guts are not used for. As can be seen above in the citation from Fonseca-Azevedo and Herculano-Houzel (2012), great apes who eat nothing but raw food have large guts and bodies which are needed to consume the raw plant foods they eat but we cannot thrive on such a diet because it is not calorically nor nutritionally viable for us—most importantly due to the size of our brains and its caloric requirements.

Carmody, Weintraub, and Wrangham (2011) show that modern raw-foodists who subsist on raw meat and plants have nutrient deficiencies and chronic energy deficiencies, even though they process their foods (cooking is a form of processing, as is cutting, mashing, pounding, etc) in different manners, while females experience low fecundity. Thus, the cooking of food seems to be needed for normal biological functioning; we have clearly evolved past consuming all raw foods. So it is clear that cooking—along with meat-eating—was imperative to our evolution. (Which does not mean that humans only ate meat and that eating meat and only meat is part of our evolutionary history.) Cooking food lead to it gelatinizing which denatured the protein, leading to easier mastication of the food, which meant less force since the food was not as hard after cooking. This then led to smaller teeth, over time, which was seen in erectus (Zink and Lieberman, 2016). This was due to cooking along with tool-use: the tool-use lead to smaller particles leading to less force per bite, which eventually led to smaller teeth in our lineage.

Finally, humans are said to be “facultative carnivores.” A facultative carnivore is an animal that does best on a carnivorous diet but can survive—not thrive—on other foodstuffs when meat is not available. This, though, doesn’t make sense. Humans are eclectic feeders—omnivorous in nature. Yes, we began cooking about 1.5 mya; yes meat-eating (and the cooking of said meat) is huge in the evolution of our species; yes without meat and cooking we would not have had the energy requirements to split off from chimpanzees/great apes. But this does not mean that we do “best” on a carnivorous diet. There are about 7,105 ethnic groups in the world (Spencer, 2014: 1029), and so to say that all of these ethnies would do the same or similar, physiologically speaking, on an all-meat diet is crazy talk. The claims that we subsisted on one type of food over the other throughout our evolutionary history is a bold claim—with no basis in evolutionary history.

Another implication of the importance Marlowe attaches to bow hunting is that, rather than starting out as exclusively carnivorous and then adding starches and other plant material to the diet, ancient humans have been able to increase the proportion of meat only after newer technology had come about, a mere 30,000 years ago. Other anthropologists concur that the amount of meat in the human diet grew as we diverged from our other primate ancestors. All of this means that, first, contrary to the claims of many paleo-diet proponents, the earliest humans did not have an exclusively meat-based diet that we are best adapted to eat; and second, our ancestors’ diets clearly changed dramatically and repeatedly over the last tens, not to mention hundreds, thousands of years, even before the advent of agriculture.

The assumption that we were fully (or even mostly) carnivorous and then added plant foods/carbs is clearly false. “Fantasies” like this are “just-so stories”; they are nice-sounding stories, but reality is clearly more nuanced than people’s evolutionary and Stone Age imaginations. This makes sense, though. Since we evolved from an LCA (last common ancestor) with chimpanzees some 6.3 mya (Patterson et al, 2006). So why would it make sense that we would then, ultimately, only subsist on an all-meat diet, if our LCA with chimpanzees was most likely a forager who lived in the trees (Lieberman, 2013).

One thing, though, I’m sure that everyone agrees with is that the environments we have constructed for ourselves in the first world are maladaptive—what is termed an “evolutionary mismatch” (Lieberman, 2013; Genne-Bacon, 2014). The mismatch arises from the high-carb food environments we have constructed, with cheap foodstuffs that is loaded with sugar, salt, and fat which is much more addictive than on their own (see Kessler, 2010). This makes food more palatable and people then want to eat it more. Foods like this, obviously, were not in our OEE (original evolutionary environment), and therefore cause us huge problems in our modern-day environments. Evolutionary mismatches occur when technological advancement increases faster than the genome can adapt. This can clearly be seen in our societies and the explosion of obesity over the past few decades (Fung, 2016, 2018).

We did not evolve eating highly processed carbohydrates loaded with salt and sugar. That much everyone can agree on.

Conclusion

It is clear that both claims from vegans/vegetarians and carnivores are false: there is no one “human diet” that we “should” be eating. Individual variation in different physiologic processes implies that there is no one “human diet”, no matter what type of food is being pushed as “what we should be” eating. Humans are eclectic feeders; we will eat anything since “Humans show remarkable dietary flexibility and adaptability“. Furthermore, we also “have a relatively unspecialized gut, with a colon that is shorter relative to overall size than in other apes; this is often attributed to the greater reliance on faunivory in humans (Chivers and Langer 1994)” (Ulijaszek, Mann, and Elton, 2013: 58). Our dietary eclectism can be traced back to our Australopithecine ancestors. The claim that we are either “vegetarian/vegan or carnivore” throughout our evolution is false.

Humans aren’t “natural carnivores” or “natural vegans/vegetarians.” Humans are eclectic feeders. Animals eat whatever is in their ecosystem. Ergo humans are omnivores, though we can’t pinpoint what the “human diet” is since there is great variability in it due to culture/ecology, we know one thing: we did not subsist on mainly only one food; we had a large variety of food, especially with fallback foods, to consume throughout our evolutionary history. So claims that we evolved to eat a certain way (as vegans/vegetarians and carnivores claim) is false. (Note I am not saying that high carb diets are good; I’ve railed hard on them.)